tailcfg,tailcfg/{nodecap,selfcap}: split capability constants to their own packages (#20639)

Package tailcfg defines the types and constants used by the Tailscale
protocol, but since everything is all in one package, it’s difficult
to sift through the docs: https://pkg.go.dev/tailscale.com/tailcfg

We define and enumerate capabilities as string constants for
tailcfg.NodeCapability and tailcfg.PeerCapability. This PR extracts
them into their own packages:

- tailcfg.CapabilityFileSharing becomes nodecap.FileSharing
- tailcfg.NodeAttrOnlyTCP443 becomes nodecap.OnlyTCP443
- tailcfg.PeerCapabilityTaildrive becomes peercap.Taildrive

We originally intended for CapabilityFoo to grant an entitlement or
permission for Foo, and for NodeAttrBar to configure Bar in the
nodeAttrs section of the policy file. However, there was no technical
enforcement of this convention, so new capabilities have used the
NodeAttr prefix regardless of meaning. Therefore, this PR unifies
tailcfg.CapabilityFoo and tailcfg.NodeAttrBar into a single package as
nodecap.Foo and nodecap.Bar.

Ran `go fix -inline ./...` and committed the changes that replaced
uses of the tailcfg aliases with the authoritative ones.

Updates #20259

Change-Id: Ieb7e7e6c8247c39faf42fdf15c68cdc7c621c730
Signed-off-by: Simon Law <sfllaw@tailscale.com>